Torque generation theorem
The electromagnetic torque of the PMSM is generated by the interaction of the two magnetic fields of the stator and the rotor, respectively. The stator magnetic field is represented by magnetic flux or stator current, which is represented by the magnetic flux of a constant permanent magnet (except for weak magnetic conditions). If the two magnetic fields are compared to two strip magnets, it is conceivable that the force of attracting/repelling the magnets is maximum when the magnets are perpendicular to each other. This means that the designer should control the stator current according to this theorem, that is, to create a stator vector perpendicular to the rotor's magnetic field. When the rotor rotates, the stator current must be updated so that the stator flux vector is perpendicular to the rotor magnet at 90 degrees.
When the stator and rotor magnetic fields are perpendicular, the electromagnetic torque equation of the embedded PMSM is: torque = 33ppPMIqs (pp is the number of pole pairs, PM is the flux of the permanent magnet, and Iqs is the amplitude of the current of the intersection). When perpendicular, the electromagnetic torque is proportional to the magnitude of the q-axis current. The microcontroller (MCU) has to adjust the stator phase current strength while adjusting the phase/angle, but this is not as easy to achieve as DC motor control.
Simplify current control to create optimal FOC performance
DC motor control is simple because all controlled quantities are steady state direct current (DC) values, and the current phase/angle is controlled by the mechanical commutator; but in the field of PMSM, how can field oriented control be achieved? technology?
DC value / angle control
First, the position of the rotor must be known, which is often related to phase A. We can use absolute position sensors (such as parsers) or relative position sensors (such as encoders) and handle the so-called "alignment". During the alignment process, the rotor is aligned with the A-phase axis such that the A-phase axis is aligned with the straight axis (the axis of the excitation component). In this state, the rotor position is set to 0; that is, the static voltage vector is constructed such that the required voltage is on the d-axis and the position is set to zero, which causes the stator field to attract the rotor and align the straight axis with the A-phase axis. . The three-phase quantity can be converted to an equivalent two-phase quantity by the Clarke transform. The amount in the two-phase stationary reference frame is then converted to the amount of DC in the two-phase rotating coordinate system by Park transformation, during which the rotor position is used.
The electrical position of the rotor is the mechanical position of the rotor multiplied by the pole pair pp. After a series of controls, the designer should generate a three-phase AC voltage on the motor terminals, so the DC value of the required/generated voltage should be converted by the inverse Park/Clarke transform.
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